umm - do you mean "all in one spot" or "all at once" because bilaterally does not mean bidaily (BID). A BID injection, for example, is twice a day. Bilaterally means 20 in one muscle and 20 in the paired muscle on the other side of the body. For example, left and right biceps or left and right deltoids.
If you mean 20/20 bilaterally versus 40 in one injection site then do 20/20 bilaterially. As for the temporal (time-related) aspect, inject 20/20 bilat immediatelly PWO (or as close to immediately as possible).