I'm setting up a company for lawyers and they have a separate GST/HST under the same company. IS this possible in Quickbooks online.

Yes, you would set up separate gst/hst tax codes for each RT number.  Assuming they only charge in one province you will only need one code. You can add extra tax codes in the sales tax module.

I would then assign the default tax code to their clients when setting up the customer to avoid having to assign it on the invoice. It makes invoicing easier!

I hope that helps!