Before running an appeal - you’ll generally run exclusions which cover a range of reasons (e.g. do not mail, recent donors, those within an exclusion time frame of a telemarketing call etc). This means you may well be removing a number of donors with a high propensity to donate to that particular appeal.
In wave two appeals - the second wave provides an opportunity to pick up some of these donors that were excluded from wave 1 - as at a later point in time, the exclusion reason may no longer apply. Additionally, as each donor’s Dataro ranking is updated each week, it is possible that donors who were unlikely to give at the time you made your wave 1 selections are actually more likely to give at wave 2.
One way of adding these donors to the campaign is to print a little extra of the wave one packs to accommodate this later mailing, and mail these extra donors the wave 1 pack, at wave two timings. They just need to be included as an extra pack type in the wave 2 mailhouse brief, and can be included in the same mailfile - with a flag to identify they’re to receive the wave 1 pack.
Once you’ve done this once or twice, you’ll get an idea about the number of people falling into this group, and be able to base your extra print volume on this.
There is very little additional work involved to do this, and very worthwhile considering, particularly if you see suppressions due to recency of last gift (those who gave very recently), or who are suppressed because of other time bound reasons. Often this cohort of donors responds very well to the appeal.