I have both ProTools and Logic. But currently I've had more practice with using Logic, but I like some of the Virtual Instruments ProTools offers. I'm guessing it's possible to use those instruments in Logic.. So, if it is possible, how can I do so?
I use Soundflower for routing the awesome instruments from Logic into Live. Usually, I just setup a MainStage for it, since I don't need any of the DAW overhead. You can do the same thing for ProTools to Logic, I believe.
Soundflower is a virtual patch bay within your computer that shows up as new outputs and inputs. I like it because it is fairly seamless once it's setup.
It's a tad tricky to setup, but what in audio land isn't? My recommendation is to use 256 as your buffer size, and get used to the occasional pops and clicks.
If you want the easy / tedious solution, use Audio Hijack and record your PT riffs.
The effective answer is, it depends which plug-in!
Check out this article. If you're working with DSP-based TDM plug-ins, it looks like you can use these from within Logic, for example. It even sounds like Logic will import the plug-in settings across from Pro Tools.
Sadly, if you're working with the more common RTAS or AS format plug-ins, then you cannot use these directly from within Logic.
Many plug-in manufacturers, such as Native Instruments provide their plug-ins in multiple formats (VST, AU and RTAS), and Logic can host AU format plug-ins. I expect most plug-in creators do something similar.
Otherwise, you're left with using one of the other methods suggested, such as ReWire, or bouncing down the audio and importing it into Logic. These methods will be fine, but are unfortunately less flexible and more time consuming. ReWire is great, but you'll need a hefty machine to run both DAWs simultaneously to a decent capacity.
Here is what I have done with some succes in the past: Set up two interfaces on one computer and plug the outs of Pro Tools interface into the ins on the Logic interface. Then I routed midi tracks in logic out to the IAC bus, and in Pro Tools I created instruments that were fed by that same IAC bus. I set logic to output MIDI beat clock on the IAC bus and Pro Tools to sync to beat clock on the IAC bus. It takes a few seconds to lock up by it works.
The timing will be a bit washy, s for best results, when you're ready export the midi tracks from logic, import to pro tools, render as audio and import that back to Logic.