Believe it or not, this would actually work fine. Give them the best HD of the two, best skill points, all class abilities and class skills, and the best saves. You end up with a gestalt character with lower HD, saves, and BAB than the rest of the party
but more class abilities. Does this sound unbalanced to you?
XP: Normal levels: Multiclassed levels
1000 2 1/1
3000 3 2/2
6000 4 3/3
10000 5 3/3
15000 6 4/4
21000 7 5/5
28000 8 5/5
36000 9 6/6
and so on...
So while the other characters are 9th level, one of their companions, the fighter/wizard, has 6d10 hp, +6/+1 BAB, +5/+2/+5 saves but also 4 fighter
bonus feats and can cast up to 3rd level spells. Not so different from a
fighter 4/wizard 5, you might say, but at 20th level the standard multiclasser
is 10/10 and this version is 13/13.