Did a little testing in Shock Force 2. It gets a little weird.
Used a 6 man U.S. engineering squad with 6 demo charges. The full squad used 3 charges against a building. Down 3 charges, they use 2 for the next blast. They have no problem blasting with their final single charge.
If you split the squad, you wind up with two 3 man teams, one with 4 charges, the other with 2. Either 3 man team will use 1 charge.
If you split a squad, use 1 charge, then recombine, the full squad will use 2 charges next time.