Hi, fashionmama! I agree with soapbuddy, don't let one layer fully harden before adding the next layer. You want to pour your next layer while the first layer is still warm but has a skin thick enough to support another layer. I check to see if the first layer is ready by gently blowing on it - if I see ripples on the surface of the soap, it's not ready for a new layer yet and needs a little more time. If you wait too long to pour a new layer, the layers can have trouble adhering. And don't forget to spritz with rubbing alcohol between pours!
I haven't worked much with the shaving base, but I would think that you can work with the low sweat and shaving bases pretty much like you would a regular melt-and-pour base. Some folks like to add clay to the shaving soap base to give it some extra slip.
Hope that helps! Have fun!