Don’t forget this the Imperial Guard and the methods that each Regiment organizes itself is based on their own preferences.
One simple way is to say that your Regiment has a policy of always having 2 Walkers active on the field, which is done by moving one squads Walker to another (and with that squads Operators, if any, performing a different function). In this case half the Regiments squads will have 2Walkers in them while the other half does not, this can even give some role-playing ideas for certain scenarios and adds a small bit of uniqueness to the Regiments style.
Another way is to say that the other Operator’s squad was wiped-out and he has now been instated with the remaining PC’s squad. It’s a common practice in the Guard to merge both squads and Regiments when they fall below half-strength, and there’s nothing in the rules saying that all the PCs have to be from the same squad (and when Hammer of the Emperor comes out maybe not even the same Regiment!). In fact the entire idea of “Kill Team” is to yank individuals of great/promising skill and put them in a single squad to do suici~ I mean “heroic missions of strategic value” .
I believe part of the reason for the whole one-per squad deal wasn’t so much to prevent PC shenanigans, which will occur regardless, but to prevent the PC’s from simply relying on the NPCs to do all the work. If every Guardsman in Hunter-Killer Regiment had a Hellhound or Walker it might mean that there are ten of thousands (if not more) of `em running around just from one Guard Regiment…and that is a lot of firepower for a single group (and any situation that would warrant it would be spell out as TPK or FUBAR).
…Plus this is the Guard, the group that has a hard time making sure that there is a lasguns for every Guardsman! Like fraking hell they would be able to make sure EVERYONE got a vehicle for every single engagement, it would be a logistic nightmare for the Munitorum anyways.
Though if the Regiment had the “Well Provisioned” Doctrine I might see it as possible…but I digress.