Combining PDFs to one PDF is easy with DT, but to my knowledge there is no simple way of combining PDFs in DTTG. When required I use the built-in PDF editor and copy/paste another PDF, and presumably can work out a shortcut to do the same.
Is there an easy way to combine PDFs in DTTG comparable to DT? If not, might I suggest this as a feature request?
I don’t see a way to do it in DEVONthink ToGo, but I notice a number of third party apps for IOS do it. If you need this capability now, it’s possible.