I don't think it is a good idea in any case. Seems to me the polarity of the secondary winding (24 volts) is not necessarily the same. For that matter, the primaries (120 volts) have 50-50 chance of being opposite polarity if on different breaker. You could be applying 48 volts when both timers power the same station, if my thinking is correct.

I'm not clear on what you want to accomplish. I'll describe something I did, see if it could apply. This was in an established vineyard with a range of block sizes, and hence flow rates. The task was retrofitting a timer and solenoid valves (and other entailed improvements) to the drip irrigation. 11 blocks in all, flows from 9 to 70 gpm, and a moderate elevation range.

The pump could obviously handle combinations of smaller blocks. The pump was set up with pressure tanks and a pressure switch control. So, a very small flow is not a problem, maximum demand just has to be within capability of the pump. I knew the range of operating pressure needed and tested for pump flow at a few points in that range. (No effect from tanks at a constant pressure.) This is the pump output curve with depth to water factored out. Take home message is: don't ask for more than pump can deliver.

Knowing pump capability and individual block flows, I considered which blocks could be combined AND could tolerate similar scheduling. Each block also had to have a single station. So, vineyard blocks 1 to 11 were assigned timer stations 1 to 11. Station 12 kept as spare. Combination (3 + 7 + 9) is station 13, (4 + 5) is 14, (1 + 6 + 8) is 15. Happened to also need a master valve after the filter.

I installed an 18 station timer, leaving a few spare stations. Ordinary timers can handle a master valve and just 1-2 additional solenoids. Aside from the timer circuitry, the supplied transformer is a little small for 3+ solenoids. So...

I removed the internal transformer from the timer, mounted the timer, and mounted a fairly large electrical enclosure just below it. Mounted a 75 VA, 24 volt "control transformer" in one corner of the enclosure. Mounted some other electrical that doesn't pertain here. Brought 120 volt power in (for transformer) and made conduit connections to the timer, both to the station wiring area and to the former transformer mounting hole. Second connection was for 24 volt power to timer, as station wiring area would be crowded.

Now for how the valve combinations are handled. "Ice cube" size 3-pole relays are also installed in the enclosure, one per combination. The 24 volt actuating coil of each relay has two terminals. One for each coil connects to the 24 volt "common" of the transformer.

(Which 24 V. terminal on transformer is arbitrary, just be consistent with timer "common" polarity. That is, test continuity from "common" to the right power in terminal.) Jumper from one coil common to the next. The other coil terminal connects to the designated timer station for the valve combo being switched, ex: stn 13 to other coil terminal of "3+7+9" relay.

The switching part of the relay has "common" terminals (3 in case of a 3-pole) and "normally open" and "normally closed" switched terminals. The (3 in this case) "common" terminals are all wired to the other (call it "+") transformer terminal. Valve wires 3, 7, and 9 are connected to the corresponding timer terminals AND a second wire goes from the timer station terminal to a "normally open" terminal on the relay. Thus, valves 3, 7, or 9 can be turned on individually, OR station 13 turns them all on. Other combinations are handled on other relays the same way. Only one station at a time is on in the timer.

Note: A similar approach (relays) can be used to perform a second function needed for just some stations, such as running a second pump, running chemigation, bypassing a pressure switch, etc. This is not going to be a common issue in home landscape irrigation.

Mark,

I know you have no idea what irri guy said but anytime you use two different clocks it is very tough unless you know what you are doing. I think I know what you are tring to accomplish and the best thing you can do is get a clock with the correct number of stations with atleast program A, B, and C and do it that way.

sprinkler medic

If the goal is implementing more complex combinations, we can work on from first post. If this is a question of station count and/or more versatile scheduling options, a single bigger/better timer is in order. Apologies if I went off on the wrong path.

Mark, if you still have questions or a different issue, we probably would need more specifics on what you are wanting to accomplish. Different timers certainly can offer different, perhaps more appropriate, scheduling capabilities. Mainstream timers operate one station at a time and may offer multiple scheduling options for each station. Just not overlapping.

Aside from problems arising from electrical cross-connection, the usual issue with multiple timers is keeping them coordinated or not conflicting over limited water delivery capability.