Yes, two letters from science professors is the requirement. However, you can get around this and get an employer letter if you have been out of school for awhile. But it seems that you will need to get one more science professor since you are still in school. You cant skip this, its seriously not only a requirement for TMDSAS, but each school that receives your application will pretty much throw you out of the pile because you wont be meeting their minimum requirements.
This seems to be all TMDSAS says on the subject:
LETTERS OF EVALUATION
Applicants are required to submit one the following options. You may also submit one extra individual letter.
Two individual letters of evaluation
OR
One Health Professions Committee Letter/Packet
Once you have completed this section, you should have your letters delivered to TMDSAS. TMDSAS accepts letters from Interfolio, Virtual Evals or regular mail. Make sure that your TMDSAS ID is entered into those systems so that we can match your letters properly.
If there is no formal Health Professions Advisory Committee or Letter Service at your institution, two individual letters of evaluation are required. Evaluators should know an applicant well enough to evaluate him/her both academically and personally. Be sure evaluators include the applicant's full name and TMDSAS ID number on evaluations. Individual evaluation letters should be accompanied by the TMDSAS Evaluation Form (with the waiver signature line completed) if the student is waiving his/her right of access to the evaluation letter.
Evaluators who choose to mail in their letter MUST mail letters to TMDSAS in an official school or business envelope. The letter MUST contain the evaluator's name, title, address, phone number and signature. The letter MUST be sealed in an envelope with the evaluator's signature across the seal. If TMDSAS receives an evaluation letter without the TMDSAS Evaluation Form, it is assumed that the applicant has not waived his/her right of access to the letter.
http://www.utsystem.edu/tmdsas/medical/application-instructions.html#eval_letters
You might check with the individual schools, however, to see what they say.