It is a great question. You can, but not easily and/or accurately (e.g. use a splitter, use REW..). Hate to say it, but I would suggest you trust Audyssey and its calibrated mic, and don't worry so much about your own spl meter that will most likely not as accurate as Audyssey and mic for this
specific application.
I understand others may disagree, but I can tell you I have spent tens of hours researching such related topic and have been using REW/Umik-1 mic to experiment, verify as well as having ran Audyssey numerous times on at least 3 Denon AVRs and 2 Marantz AVPs. I don't just go by hearsay from forums, Denon and/or Audyssey. I am sticking to my belief, based on my own experience and readings.
You can find out for yourself instead of taking other's words for it, by joining the club and invest on a Umik-1 mic and spend some time experimenting with REW. There are a few threads where you can see people findings, using REW.
REW is free software for room acoustic measurement, loudspeaker measurement and audio device measurement and analysis
www.roomeqwizard.com
The UMIK-1 is an omni-directional USB measurement calibrated microphone providing Plug & Play acoustic measurement. From speaker & room acoustic measurement to recording, this microphone provides low noise and accurate results you can rely on.
www.minidsp.com
There are a few threads where you can see people's findings, using REW. I can provide links if you are interested..