I realize this is a an old thread but I thought I would make a contribution nonetheless. I used to work at Boardworks Tech Shop in Bellingham, Wa. Johnny Lupo, the owner, makes killer custom splits there.
When considering splitting a board, we always told customers that a board should be in at least a 7/10 condition. So no major delams, big base gouges, etc. However, if the board's base still has a good amount of life in it and it is not "flexed out", an old board that needs some base work can be a great board to split. In terms of how the flex changes, you usually lose roughly 1 flex rating (a 7/10 becomes a 6/10).
As far as splitting a board yourself goes, you definitely want as thin a blade as possible (while still making it through the metal edge) or better yet a water jet (hard to get a hold of one though). You also want to measure, remeasure, and measure five more times when figuring out your center line.
If you end up deciding you don't want to risk ruining your snowboard, Johnny at Boardworks starts pricing at $400 for a basic splitboard build and you can mail your board in. A basic build includes a full tune, board cut & seal, & Voile hardware installation. The price also includes return shipping to you. It is a pretty good deal if you are looking to have a custom split in your quiver. More info here:
https://boardworkstechshop.com/collections/splitboards-custom-builds/products/custom-splitboard-build-service-by-mail
That being said, building your own split is awesome and I say go for it. But maybe practice with a couple throw away Goodwill boards first. That way if you mess up your cut it isn't a big deal.
Also, not just looking to plug my old shop! I just moved back to Spokane and am stoked to get out touring! Hope to meet up with some of you soon and get some laps in! Cheers! Hope the info helps people looking into custom splits!
