Atenolol can and often does have an additive affect when taken along with other beta blocking meds.
In my case, even though the Atenolol was only 25 mg, it was enough to cause several heart pauses (flatlines) of 3 to 4 second durations detected via an event recorder.
Regarding the interaction between diltiazem
and Tambocor, this is from drugs.com:
together with flecainide can increase the effects of either medication.