BIAB with alpha and beta rest

I want to do a marzen that calls for mashing at one temp for 30 min (140*) and then a higher temp for 30 min (158*).

Can I just do it like a single infustion mash at 140* for 30 minutes and then raise the temp with the burner to 158* and mash for the additional 30 minutes? I have a second pot I could heat water in to add and try to raise the temp.

Any help is appreciated....just my first lager and I want to get it right.

Yes, but be careful not to burn the bag/grain. And stir a lot.

Probably better to use the second pot for heating the water. I'd be very worried about melting the bag.

The kettle is much bigger than the bag, so it does not rest anywhere near the bottom. I was thinking about adding heat and warmer water to get to the second rest.....or just heating it to 158 and holding for 30 min.

heat it... no worries about melting the bag as long as it's not sitting on the bottom. even if it was as long as you stir non-stop it won't burn. OR... a better way to raise the heat is by doing a decoction.. I've done it that way for a hefe and it worked great.

I do step mashes fairly regularly with full-volume BIAB on propane. If your bag does not sit on the bottom I would not worry about it burning. I use a wire cooling rack as a stand-off on the bottom of my kettle.

The biggest challenge I have found is not overshooting the temperature. The kettle retains heat and continues raising the mash temp after you turn off the burner. Stir a lot to get an accurate temperature reading, and turn off the burner about five degrees before you achieve your desired mash temperature. If that does not get you to temp, turn on the burner for about 30 seconds, stirring, and try again. It also is not a bad idea to keep some extra cold water ready if you overshoot, and resign yourself to a longer boil.

infusion, decotion (boiling about 4-5qts in either case), or the burner would all work. if you use the burner you may want to shorten the beta rest tho since it'll take some time to get up to temp.