One option is to cascade the digital output of the Elevenrack into your other ProTools LE interface. In the User Options and change the Digital Output from “Rig Out” to “Split In/Out”. (see p.93 and 95 of the ElevenRack manual) In that setup, you’ll have all the other interface’s inputs available, and your SPDIF will give you a mono sum of the Rig Out as well as the dry guitar on the other channel. You’d record the SPDIF left and right to two mono tracks in ProTools.
The other option is to switch the current playback engine setting to the Elevenrack, and then reset your IO Setup. This method will give you all the inputs from the Elevenrack as well as the stereo rig output, but none of the inputs from your other interface.
When you have two ProTools LE interfaces attached to the computer, you can switch which one is being used as the Current Playback Engine from the Setup>Playback Engine menu in ProTools.
To reset your IO Setup, go to Setup>IO Setup, Input tab. Select all the input paths, click Delete Path on the screen, and then click the Default button on the screen. Once you’ve reset your IO Setup for the Elevenrack, you’ll have the unprocessed inputs as well as the rig output available as the input to a channel. (see p.85-86 of the Elevenrack manual)
It’s useful to get your IO Setup correct for each interface, then then use the Export Settings buttons to save these settings. You’ll be able to import either IO Setup into existing or new sessions.