Ok, in that case I will just stay with fighter until level 10 or 11 or whenever that second prof thing is.
Second bonus is at level 12. You can then put 5 proficiency points in weapons and you get 1 point in str and 1 in con, plus a fighting tattoo.
To be honest i wouldn't recommend going that far, or taking the Fighter bonus at all, UNLESS you are planning to play Fighter until the end, or you are planning to grind for xp. The Mage tattoo is better than the fighting tattoo, plus if you are going Mage you won't get to use the fighter tattoo. You can give it to Dak'Kon but still.
More importantly though, it takes a shit ton of xp to get to 12 (or 13, where you can get an extra half attack i think, so 13 fighter and rest mage is the most munchkin combo in the game). There's no way you can get that by playing the game organically, you'll have to grind Undersigil or you'll have to play as fighter for at least 70% or 80% of the game before you can switch back to mage.
A good alternative to Fighter 13 and rest Mage is Fighter 9, Thief 10 (you can get an extra attack i think and the xp to get there is negligent) and rest Mage, with the Mage bonus instead of the Fighter one. Your starting stats can be something like this:
STR 9
DEX 12
CON 14
WIS 16
INT 13
CHA 11
Constitution bonus over 14 is retroactive, but you get 10 extra HP points from going to 9 to 14 at character creation you can't get back by raising Con later on, and starting with 14 puts you in a position where you can raise it right away as needed. From here, i would go Fighter up to 6, Mage up to 7, Fighter up to 9, Thief up to 10, then rest Mage.
As far as stat points i'd go like this. First goes to Dex, raising it to 13 while you are still in the Mortuary so you can get the neck snap check on the third floor. After that, you can can put the next 2 to Wis, but no more, since you can get a total of 8 points (+7 from quests, +1 from the Mage bonus at level 12, -1 from the Gordian Knot, total 25 which is the cap), or raise Con or Dex at your discretion (there's some Dex checks that go up to 18 related to the Thief class. Not important but you can just play with that while you are leveling Thief. Keep in mind you get +1 point from quests so don't go over 17 in Dex, since 18 is the sweet spot, anything over being way too much of a waste). For the record, you don't really need to just boost Wis to the max right from the start since any xp you lose in the beginning you can make up later on with a single trip to the Undersigil, so you can keep Wis to 16 and boost the other stats for a bit. With a tattoo of +2 plus the +1 bonus from talking to O at the beginning will allow you to get to 19 so you can do the Dak'Kon upgrade chain right away, and you can just keep it at that for a bit). For Str i would just use the Str spell early on, which can last quite a bit, and only raise it once you switch to Mage permanently if you want, because as Mage you only get a 1D4 bonus as opposed to a 1D8, all though that's relative since eventually you'll just be using spells rather than go melee. If you want to be anal about your munchkiness just make sure you have 18 Str before getting any quest bonus so you can skip from 18 to 19 or over in one swoop.
Ho, as for Cha, get a +2 tattoo asap, and use the friends spell as needed. 13 Cha is good enough for most of the early portion of the game, with only a 16 check in Dead Nations, plus a couple of 16 checks for better quest rewards that aren't critical. Once you get to the Lower Ward, get the +3 points from the quests that are there (check a guide if you want) and with 16 Cha plus friends spell you should be good. Later you'll get +2 from the Gordian Knot and +2 from the glass eye that drops from Ravel. The highest check in the game is 24 but with a basis of 18 you can get there with a single cast of the friends spell.
BTW, as far as the combat stats go, Con is crazy good. At 25 your regeneration rate is so high at points you can gain HPs faster than the enemies can take away. It's a pretty ludicrous stat.