Unfortunately, no. Say you have an 8GB GPU and a 12GB GPU installed on your computer. The total available memory will not be 20GB, i.e. the 8GB GPU will not be able to use the 12GB GPU's memory. This is a limitation of current GPU technology and not related to Redshift in particular. We, therefore, recommend users combine videocards that are fairly "equal" in terms of memory capacity. Having said that, Redshift supports "out of core" rendering which helps with the memory usage of videocards that don't have enough VRAM (see below). This means that, in contrast with other GPU renderers, the largest possible scene you'll be able to render in the above scenario won't be limited by the system's weakest GPU.
See https://support.maxon.net/hc/en-us/articles/1500006456521 for full details.