Yea, I meant the second step being in the kettle - just may try it as an experiment, but not likely a standard operating procedure for the future.
So the idea is to drain off via a batch sparge a bit early so you can bring the temp back up in the kettle?
I don't see any problem with this - depending on your recipe, there may actually be hardly any starch left to convert anyway (as some have noted), but conversion will continue until either the starch is spent or the enzymes have denatured. I think if your'e careful, you can hold a roughly 158 degree temp for long enough, but it is easy to overshoot on the way up, so I'd cut the heat a bit early.
For the record, I recently ran a constant 160 degree mash in an attempt to get a more full-bodied (dextrinous) session IPA. It seems to have worked quite well - with a 1.046 OG finishing at 1.015 (3.9% ABV). Both beta and alpha amylase are active at this temp - for a while. The beta can't hold out long, but while it's working, it is working hard. I clearly had plenty of fermentable sugars in the final mix. (The "scary high" mash was inspired by the Lagunitas IPA recipe - of course I could have just used dextrine malt or maltodextrine, but I wanted to try to achieve this through the mash process itself). In short, if you do overshoot 158 by a bit, it probably won't be the end of the world - and in your case, the beta amylase has already done its work, anyway.