Here's what I've learned about splitting cards.
1) After you tap the card corner to get it started, look at it and then decide which way you want the split to go. (Thick - Thin or Thin - Thick). If it doesn't look like it will split the way you want... remember you have 3 more corners to try to get it started the way you want. When starting out, bang all four corners and then decide which corner looks best.
2) Always pull "thick side away from thin." I give a fairly quick pull from the start corner towards the corner opposite it. You'll have to use trial and error to discover what works best for you.
3) Flatten out the split cards overnight before you glue them. I use two glass welding faceplates and a couple strong spring clamps.
4) After you glue your gaff together, flatten the card overnight. This makes the gaffed card act and feel like any other card. Don't play with the gaff until it has been flattened overnight!
5) I like to use 3M Super 77 spray adhesive. This stuff really holds.
6) If you are making gaffs, it is easier to make a hole bunch of each card gaff at a time. For example, when I was making Hofziner gaffs, I got two forcing decks of the two required cards. This way, I didn't have to ruin a whole deck to get the card(s) I needed. Also, I had plenty of extra cards in case I screwed the split up.
7) Split cards while watching T.V. I can sit in front of the T.V. and split a whole deck in about 25 minutes. If I don't sit in front of the T.V., I get bored after about five minutes.