Not at the same time, no. What Japican and I did was send each other the vmf file through google drive. An alternative to that is having one person open the map, then his friends using teamveiwer to control some aspects of his computer. How much lag you had would depend on your connection type.

at valve software there are multiple mappers working some working on desing some working on entity framework.thats how ı do by myself too ı make the enviroment first then entities