If your looking to build a larger machine, like you seem to be then Personally I'd sell them and start again.!!

You will lose very little money as they are brand new and they will be perfect for a smaller machine. Where has if you continue and use them you will only lose money.!

To use them now means you buy or build a 44-48V supply so this will be no use so wasted money at a later date when you WILL need new drives either because like John says they Die from the strain or you want higher speeds or torque.
Jonathan's suggestion to gear them is OK and a good work around but it is just that a work around and less than ideal and does come with cost's.?
Lower resoultion and lower torque plus more expense of belts and pulleys (Thou I'd still use them anyway but for different reasons!!)

Sorry to say this but I suspect these drives are cheap versions of real Leadshine 542's so the chances of them giving up the ghost under strain is quite high and this is why I say cut and run now before you waste money and get frustrated with the compromises.

The drives and PSU plus good quality control box build practice is one of the most important areas for a hassle free machine skimp here and you will regret it.!!