double issue; combined issue
Used for magazines, journals, or other periodicals that combine two or more regular issues into one, often for consecutive months or special editions.
The January and February issues of this magazine are a combined issue.
It's common to have a double issue around the New Year holiday.
号 is a general counter for magazine issues; 合併号 specifically refers to a merged issue.
Compound of 合併 (merger, combination) and 号 (issue, number). The reading がっぺいごう follows standard on'yomi.