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If you want a thick resist layer, you might try spinning multiple layers. Then you'll find that this does not work very well, because the solvent in the second layer dissolves the first layer. It's a mess. You will get a much better film if you spin a thicker solution, just once.
To thicken an e-beam resist such as HSQ, we simply bubble nitrogen into the bottle, through a tube. Do not heat the resist above room temperature. It takes all day (about 6-8 hours) to bubble away 200 ml of MIBK in a bottle of HSQ. This same trick can be used for positive resists, such as PMMA or CSAR.
On the other hand, it is much easier to buy the dilution you really need, so I have to wonder: Why are you doing this? In the case of HSQ, thick formulations simply cost too much. Maybe you need just a little bit.
A thickness of one micron is definitely the limit for HSQ. Beyond this, the film will crack badly. Two microns of HSQ is going to be a disaster of cracks. Exposing the resist results in 6% shrinkage, so it's going to be even worse after exposure. Are you trying to make a waveguide out of HSQ? This will not work.
Thick PMMA will form stress cracks at the corners of shapes, if you develop it with MIBK-IPA mixtures. To avoid this stress, develop instead in IPA-water (3:1 or 10:1), optionally at lower temperature.
CSAR over 1.5 um will form bad stress cracks at corners. Will IPA-water help with this? We don't know. Try it and let us know!