[SixOfSpades]

If you really want a good THAC0 with a Fighter->Thief, don't Dual until you hit Level 21. Level 21 (or thereabouts) is where all classes' THAC0 progression stops, and since Warriors have the fastest THAC0 progression, it's to your advantage to have 21 Warrior levels.

For Multi-classes, the game calculates the THAC0 that each half of your character would have, and grants you the better one.

Dual-classes are trickier: When you Dual, your THAC0 progression becomes that of your second class (in your case, Thief). When you come out of the Dual (in your case, when you hit Thief Level 10), the game compares the THAC0 that each half of your character would have, and grants you the better one.

A Level 9 Fighter would have a THAC0 of 12, and a Level 10 Thief would have a THAC0 of 16. So, when your Fighter half reactivated, your THAC0 was 12. Ever since then, your THAC0 has been improving at the rate of 1 point every 2 levels (which stops at Level 20 or 22), so at Thief Level 20, your base THAC0 is now 7.

Now, if you had postponed Dualing.......
A Level 21 Fighter would have a THAC0 of 0, and a Level 22 Thief would have a THAC0 of 10. So, when your Fighter half reactivates, your THAC0 will be 0. You would not gain any more THAC0 progression from your Thief side, so your base THAC0 would now be locked at 0.