There is no catalog sync in LR.
This must be done manually, or better yet, don't have photos that are imported into both LR5 and LR6. Even so, work done in LR6 is not completely transferrable back to LR 5, so if you're not 100% sure you're going to pay for LR 6, you might want to create some un-important photos to play with in LR6.
I already bought it, I was just thinking that since it synchronized my catalogue when I installed it, it might be possible to do it manually again. The idea being that if I'm inporting or exporting something tht takes a long time in LR6, I could carry on working on LR5 if the two were working together. Just a thought, sometimes I reall would like to work on more than one window at a time.... but it would probably mess things up, right? The alternative is just to uninstall LR5 once everything is working well?
When you are importing or exporting, you can still perform other actions in Lightroom (such as Develop or add keywords). No need for two windows.
The alternative is just to uninstall LR5 once everything is working well?
Alternative to what? It's your choice if you want to uninstall LR5 or not.
I don't know about LR6, but LR5 becomes pretty slow when performing multiple tasks. Now I hear LR6 is faster, so that may be ok. Thank you!!
I doubt that the speed improvement in LR 6 applies to exporting or importing. As far as I know, the speed improvements are in the Develop Module.
And even if you were able to open two windows, importing or exporting in one window would slow down whatever is going on anywhere on that computer, you don't magically get extra CPU power by having two windows open.