if they're decent, thick, plastic, you should be able to either drill a hole the correct size and get a threaded plumbing fitting to cut it's own threads in the plastic, or use plastic fittings that have a large nut that can be tightened onto a bed of silicone to seal around the fitting

multiple rain barrels can be cascaded by putting them on higher to lower stands ... if you really want to do it, dig a cistern as big as you can, like a covered pool, get a small submersible pump, like a fountain pump, for watering

Bill