How to split 5V USB input into 4 rails (+/-5V, +/-15V)? Problem with combining two virtual grounds
Alternative solution:

I generally think virtual grounding adds more issues than it resolves. As such I highly recommend a different power splitting approach that maintains the original ground. 

For example....



Of course the above is only one example. 

How you actually break the rails up depends on how much current you need from each supply. If the negative rails need higher currents then using separate boosters may be required.