Sorry, but specification forms are almost always designed to print one document or customer or vendor per page, rather than multiple on a single page. Exceptions are the 1099 forms in Payables and Labels in Order Entry. Checks and/or advices are definitely not multiple vendors per page.

Perhaps instead of printing to a laser printer and standard paper size, you could email the advices? Or print to an electronic file such as a PDF and then deal with printing from that file? You can specify a custom page size on the advice spec when editing it, from the File / Specification Options function.