It turns out that the way hit points for multiclass characters was intended to be handled was never clearly stated in the books. It wasn't until Dragon #243 (printed in 1998!) that we got any kind of answer.
What I will say about multiclassing is, if there's any class that you really shouldn't single-class, it's Thief. Given the benefits of very high Dexterity (where an 18 lets you basically function as if you are +1 level!), you likely should be considering Elf or Halfling anyways, and being down a level compared to a single-classed Thief is well worth it when you consider the potential improvements to survivability, utility, and combat prowess. Most Thieves aren't giving their party much benefit from their Thieving abilities at low levels anyways, with almost any check starting at under 50% at 1st level, and failed checks often leading to doom for the Thief. In combat, the only character worse off than you is the Wizard. Backstab is an unrealizable dream (1), so your best bet is to grab a shortbow and fire twice a round for your d6 damage.
The benefits of a second class here are huge- if you're part Fighter, you probably still don't want to go into melee, but you could upgrade that d6 bow shot to a d8, have likely better hit points, and eventually better saves. If you're part Mage, well, you're not going to be hardier, but the utility gain is immense for your role- eschew armor for better Thieving abilities with Mage Armor, create distractions with illusions, turn Invisible, be able to use scrolls or wands much faster- there's a lot to consider.
And then there are some strange multiclass combos that will cost you that +1 Dexterity- Gnomes can be Thief/Priests, which suddenly adds a few healing spells to the party (always welcome) and some crazy things can be done if you're a Mythos Priest (2). And the Complete Bard's Handbook offers some Thief/Bard options, which not only results in extra utility, but a second source of points for Climb Walls, Detect Noise, Read Languages, and Pick Pockets! I'll mention Thief/Psion, though I've never seen one played.
(1): depending on how strict your DM is with the rules. I've encountered (even on this forum) people who claim Thieves are backstabbing machines, constantly ganking enemies, despite the potential minefield of rules that the DM can easily use to prevent this ability from functioning. YMMV.
(2): in the past, when I've talked about this, I've encountered some DM's who claim that this shouldn't be allowed, as, in their minds, a Mythos Priest is some kind of specialization, and Cleric =/= Priest. It's worth noting that in later TSR books, like the Complete Planeswalkers Handbook, it specifically states races can be multiclass Priests, not Clerics. Again, YMMV.