As noted by others, the two team hit scenario has been used by many a gm in the past. Traditionally the PC's are the decoy, while the more professional stealthy group steal away the gizmo while the security forces go after the PC's, but having the PC's interrupted midrun by a boomtastic shazam-over-subtlety group could be fun.
For me, the most important thing to make the situation resonate is to give the Johnson a logical reason for hiring two teams, especially if he knows the NPC team are going to large it up, set off all the alarms and make it hard for the PC's. Whatever logical reasoning you give him, he should certainly play it suave when the PC's come to him screaming for blood, e.g. "I thought you might need a little help getting out, so I sent in a decoy. You didn't mind, did you?"
The one assumption you are making in this scenario is that the PC's don't trip any alarms themselves before the subtle-as-C4 team arrive, are you going to fudge the rolls so this happens?
The other assumption is that the PC team and NPC team are going to engage with each other. Once the alarms are set off it means the HTRT forces are on their way, and this is usually the cue for runners to grab the gizmo and get the hell out of dodge. Unless the gizmo is a big old prototype that the NPC team spot when the two groups cross paths, neither group really has a reason to engage the other.
Anyway, sounds like a good plan. Hope it goes well!