From a very basic perspective, according to NCCI edits, they can be billed together with no modifier. HOWEVER, this is a loaded question and there are other factors at play. Some of the key ones are: these are time based services, the eight minute rule, one-on-one direct patient contact, whether the payer has limits to the # of units/time for therapy per day, modality limits, etc. A payer might have made their own guideline regarding whether these are reportable together or not which might differ from NCCI.
The documentation has to be very clear as to what is being performed.
Sounds like you may want to look into a basic PT billing webinar or course (if you're doing outpatient PT?)