The DP34 is not compatible with the DPP Twin (there are some documented exceptions) so you could get a DP Twin and use the DP34 as you describe.
Or swap for the DP Twin and get a DPP33 (also not compatible with the DPP Twin) instead of the DP34 which will save you from having to run another line to the 722. It will also allow you to upgrade the single-tuner receivers to dual-tuners (a 211 to a 612 will keep you at the lease limit of 4 TV outputs).
If you have an original Dish 1000, you could replace it with a 1000.2 which has an integrated DPP switch with 3 ouputs.
A Dish'n It Up upgrade through Dish will include the LNB and/or switch changes that may be needed as long as you're within the leased TV output limit.