I found it easiest to finish both paint colors, then do the final wet sand and polish. Be ready for some very cautious tape removal, though. On the new paint, with the time the tape is on the surface it gets quite a grip. Use a cloth wetted with mineral spirits to rub the tape as you remove it, and go slow. This will soften the adhesive and prevent the new paint from being stressed and tearing. I found this out the hard way, but my wide trim covered the damage that i had to touch up.
Also, an old body trick is to remove the tape on the bias, _____/ <--- pulling at an angle to the line of the edge.
By wet sanding both colors at the same time for the final it will also help to eliminate edges left hwere the tape met the paint. Then your pinstripe can be applied and smoothed in as well.
Alternatively, if you apply a strip of tape the width of the stripe, then paint up to it in both colors, then remove it and tape the outside, the stripe can be applied like an inlay, almost entirely eliminating any raised edge. Someone else posted this method earlier for hood stripes, but ATM I can't find it for you.
Either way will work, but the most needed item besides tape is patience. Two tone and/or stripes add to the labor and intricacy, but I found the results worth the effort. Took me almost a month of 2-3 hrs a night and all day Sat. to pull mine all together from looking like this first pic.