Why not just heat the water in the same tank as you mash in? It seems like a huge inefficiency to have another tank just to heat water in.
You could heat water in your mash tun, providing it was a kettle with a false bottom (as opposed to a cooler with a false bottom) a then add grain, but then you would still need an external way of heating your sparge water.
In a basic batch-sparge system, the reason for the hot liquor tank is gravity. Your hot liquor tank (which is a misnomer, really a hot water tank) is the highest container, so you can use gravity to rack to your mash and lauter tun, then gravity again to rack to your boil kettle.
I use a very rudimentary, batch sparge system, with one tun (a 10g igloo cooler with a temperature gauge and a valve), one burner, one kettle (8g with temp gauge and valve) and a few buckets. I have to swap elevations of the kettle and tun each time I want to transfer and I cannot transfer my mash to my kettle since I need it to heat up my sparge water. I have to rack it to a bucket, then rack the bucket to the kettle after I'm done transferring my sparge water to the tun.
I've thought about another kettle, burner and tun, but not for a gravity system, so I can do two 5 gallon batches in parallel.