Pennchlor 64 antibiotic for respiratory infection
I don't know anything about the specific antibiotic, so i would check on their internet site for further details - the general rule of thumb is do not eat any eggs for 10 days after administering the antibiotic.
I would imagine that if one hen is showing symptoms of a respiratory problem, then the chances are they both have it.
Do bear in mind that a respiratory problem can be viral, bacterial, fungal or an allergy. Antibiotics will only work if the problem is bacterial (but it will help fight off secondary infections if the problem is caused by the others).
For what its worth, i only give antibiotics as a last resort, i.e. if the problem is acute / serious. Normally, i just give vitamin supplements / electrolytes in their water (boost their immune system and helps them fight off whatever "nasty" they have) and let them deal with it themselves. As long as they are feeding and acting normally, i would think that they will be ok.
Considering the situation, i reckon you have a sound plan.