No experience with this but wanted to send good thoughts that you'll get them placed together. Thank you for fostering.
My gut feeling is that their reactions would depend on the individual dogs and their actual relationship dynamic...as with the death of one affecting the remaining one. Some dogs don't seem to notice that the other one is gone and others will show signs, sometimes extreme, of mourning. Add to this scenario, though, a change in owner and environment, too...so potentially more severe. Potentially.
My first Lab Brutus came to us at 3 and never looked back...he was mine the minute I met him (in his previous owner's house). Sat on my foot and looked behind, up at my face. Bru wasn't leaving another dog but he was leaving an owner (with kid...nice people who just couldn't take Bru to Florida with them due to heat and Bru's intolerance...good relationship with Bru). So....people/environment switch wasn't a biggie. We also had another dog (and two cats) who warmed up to Bru fairly quickly so his settling in was fast.
My present girl Sunnie came to us from a shelter (stray, no background, probably at least 6) and it was pretty apparent even at first that she was needy enough to want to be an only dog. Too bad...she was also pregnant and we kept little Danny. To this day, 10 years later, I still think Sunnie might appreciate being the only dog and be one of those who doesn't notice the absence of another. Dan, on the other hand, mourned our one cat...his best buddy...for about 6 months after Charlie died. If Sunnie goes first, I'm pretty sure Dan will get even needier (nut didn't fall far from the tree).
Do they display signs of separation anxiety when one is walked without the other? Maybe "test" them with something like this if you don't already know....that might give you an idea of what impact the separation aspect of the equation will have. Change in owner/environment would have already been neutralized a bit when you took them in since they were together.
Again, hope you'll be able to let them stay together. That would be one less change in their lives.