They need to work together to make a cohesive look.

Please stop them and ask them to step back and look at it with you. Point out the differences and ask that they change anything that is already done to be consistent. Also ask that either one help by providing materials to the other so that only one technique is used or ask them to work differently in order to provide a cohesive look as orangecamera suggests.

This reminds me of 2 people doing a faux paint finish - each will have their own flair. Not a good thing.
I would give them a really nice lunch (large loaded pizza and sodas) then ask them to stand back and look at the work they're doing. Tell them nicely you appreciate their work, but it is not as even a design as you hoped it would be. Maybe the design work should be just done by the one person who's work you like the best. Maybe they have no clue the sides look different and don't mind you telling them.
After all, you are the boss, you are the customer and it is you that has to live with the look of this f/p for-ever. :)

can you post a picture?

agreed, they should be switched from time to time so their own techniques isn't as visible.