You can apply for both, but I woulnd't interview for both at the same hospital. Imagine if you saw the anesthesia program director in the halls a month after your interview - oh, what are you doing here again?...umm...interviewing for ortho. That's not going to get you in to either one. Although I know several people who interviewed for two programs at one hospital and never got found out, I would be nervous wreck wondering if they'd find out my disloyalty to the specialty. Same city, different programs would be okay though b/c I dont' think word would get around that much.
Consider applying for both, but between now and interview time, doing at least one more rotation of each, at hospitals other than the one you rotated at already, to get a better feel for the two. Then, you can choose to accept interviews in the specialty you like better. Also, the programs may decide for you - both are kind of competetive now, ortho still more so - say you apply to 25 ortho programs but only 3 offer you interviews, well, good thing you put in those 25 anesthesia applications too.
The third option if you are truly undecided when it comes time to make the rank list, is to do a surgery prelim year. Yes, this delays the finality of matching into a known specialty, and makes you have to go through match twice, but it gives you a year to try out the surgical life and see if it's for you, as well as getting to see the anesthesia residents often and check out their life a bit more. If you find yourself in the OR always peeking over the curtain to see what anesthesia's up to, you will know that's where you should be. You have to do an intern year for anesthesia anyway - although most do medicine or transitional, I am sure anesthesia programs would look at you as a super hard worker b/c you chose to do the toughest intern year of the three options - and the most procedure-oriented (an IM intern easily puts in less lines all year than a surgery intern does each month).
They're both cool specialties and it's hard to make this decision. My vote is to leave yourself many options at this point - apply to both plus some prelim surg programs, and continue to think on this decision over the next few months before you have to make that rank list.
