The sets will seemingly do 3 trips a day, so not dedicated to one peak business service to Cardiff and back.
The current timetable sees the single set lay over for seven hours during the day, enough time for an extra mid-day journey without losing the morning/evening business traffic. Add a second set to run North-South-North while the other runs South-North-South and you've got your three services a day (with one set spare).* The "active" sets will alternate between berthing at Holyhead and Cardiff each night, which will make maintenance much more straightforward than at present.
*At present the northbound service is part of the two-hourly service, but the southbound isn't, so the timings may be shuffled about somewhat.
A possible timetable, for illustrative purposes:
Set 1:
0425 Holyhead - Cardiff Central, arriving 0920
1119 Cardiff Central - Holyhead, arriving 1610
1651 Holyhead - Cardiff Central, arriving 2136
Set 2:
0525 Cardiff Central - Holyhead, arriving 1040 **
1041 Holyhead - Cardiff Central, arriving 1521
1711 Cardiff Central - Holyhead, arriving 2137
**New service. Path currently used by a Manchester-Holyhead service, which then runs to Cardiff.