Hiya,
As you say that they get along fine in a playpen i think you should just try putting them together in a completely clean cage. Make sure that none of the toys or wheel have scents on of either one and just try putting them in together - probably best to only put food and water in at first, not even any bedding, just so you can keep a close eye on them, even when they are in bed - they should just hopeully snuggle up in one corner.
For me, the split cage method is the 2nd method i try, as a lot of robos are social and do refer company, so just see how it goes
Obviously you may still need to use the split cage method if they fall out, but just see.
Tammy x