Hmm.... I've had mixed results with regards to players with multiple characters at the same time over the years.
I'll assume seeing as you all play together already your group will be dicks about this and you'll avoid the " this character sells all his stuff and gives all his money to the other character and always jumps in the way of bullets" that can ruin this.
But... in my experience this can cause problems if the team ever have to split up -- as you can wind up with 1 player with two characters having to roleplay both at the same time. It's quite odd to hold a conversation between yourself.

.
Also, inter character conflict is a fairly key part-- even if it's due to madness or possession-- of the
DH game in my experience, and you might ruin the risk of the 2 character players "ganging up" against the 1 player characters.
Biggest problem I've found with this overall is normally 1 character out of the pair becomes favoured and the 2nd one becomes just a supporting character.
...so with that in mind I would suggest :
Either full on embracing this : get rid of the guardsman straigt away and just give the ad mech guy ( for example) a combat servitor or two. Gives them the muscle they'll need and avoids any problems with character development and gives the player something to do in the combat as well.
Or maybe rotate the control of the spare characters from sesssion to session, or keep one of them at least as a
GM controlled
NPC. IN my experience, especially in
DH it's well worth having at least
NPC with the party to help steer them.
Only other thing I can suggest :
Years back I ran a cmpaign and every player had 2 characters ( 3 in the group ) each player had one fighter/warrior character and 1 tech/support character who was in constant contact with the fighters by plotdevice communications. Each techie character was assigned to a different fighter so they wouldn't have to talk to themselves. This way the figters could go into the dungeon/whatever and still be advised and aided by the smarter but crap in a fight brainy guys. Worked quite well.
Think I stole the idea from Batgirl/Oracle comics.
Migt be able to bend that concept perhaps ?
Be interested in hearing your campaign ideas though.