The last app update (test version) is about 2 months ago and they changed only „What’s new in v3.0.2 - Added Flow Cards for all measurements.“
So I guess it is not possible.
A user in the German forum said that he connected 2 sleeping mats via IFTTT.
Maybe you just write and ask him if one sleeping mat can be connected via Withing app and the other sleeping mat via IFTTT.
