You should just be able to modify the A records for domain2.com to match the IP used in the A records for domain1.com. So for example:
If this is the record for domain1
;; ANSWER SECTION:
www.domain1.com. 00000 IN A 00.00.000.000
domain2 would be
;; ANSWER SECTION:
www.domain2.com. 00000 IN A 00.00.000.000