Tizanidine is contraindicated with concurrent antihypertensive therapy. Check your blood pressure before you administer this drug, but do not discontinue the atenolol abruptly (this will cause a rebound effect.) If you are checking your blood pressure and it's dropping below 90/60 or if you change positions (like standing up) and you feel dizzy then you may need a lowered dose of Tizanidine or possibly a different medication.
Well I am not a doctor, so I am not qualified to offer a second opinion.
However I will make an observation
I think its safe to take both meds given the dosage amounts you quote in your post.
Normally Atenolol is prescribed for high BP. In this instance its taken daily or in a regular pattern as prescribed.
Sometimes if its prescribed for anxiety issues, it can be taken as required. Perhaps you should consult your doctor and consider this as a way forward.
I will say though that stopping Beta Blockers can sometimes have a re bound Tachycardia effect, at least for a while after stopping them
Your doctor seems like many doctors, once the BP numbers are good, even if a bit low, they dont really care, as they seem to have no problem once the numbers are good.
See if the doctor will begin a programme to ween you off the Beta Blocker slowly over a number of weeks?