Conventional wisdom, and the recommendation of some battery
manufacturers and "experts", is that you should not mix batteries of different ages and types.
However, there is more than enough empirical and experiential evidence to suggest that this generally accepted advice need not be followed slavishly.
On my boat
a few years back, I did the same as you are planning, i.e., increased my house battery
bank from 4 T-105s to 6 T-105s with no ill effect. Then, when it became time to replace the house batteries last year, I replaced all six, plus the two I use up forward for the windlass
I'd think that if you've been treating your four T-105s well (not too deeply discharging them, fully charging
them, maybe equalizing them occasionally) then you'll have no problem with adding two new ones.
While you're at it, be sure all connections are clean and tight, and you are using adequate size cables