A DishPro "splitter" (the first PN) is used ONLY to split the output of ONE DishPro LNB, usually on a Dish300 aimed at a wing satellite, to two different places (switches and/or receivers).
A DishPro Plus "separator" (the second PN) is used ONLY to distribute a DishPro Plus feed from a DPP44 (or from a DishPro Plus LNB when available) to the two satellite inputs of ONE dual-tuner receiver--a 322, 522, 721, or 921. All but the 721 have DishPro Plus logos on the front; I understand the 721 was upgraded to DishPro Plus by a firmware update. NO single-tuner receivers have DishPro Plus logos.
The "separator" WILL NOT WORK with single-tuner receivers because only ONE of the two outputs will accept switch (DiSEqC) commands back to the DPP44; that's OK for one dual-tuner, but NOT for two single-tuners. Though a "splitter" can be used in systems with single-tuner receivers, it's almost exclusively for sending a wing-bird signal to two different switch inputs (either two DP21s, or two cascaded banks of DP34s and/or DPP44s); it can't be used on a DishPro Plus output for the same reasons as the separator.
You can't do it now, but the ONLY way you could EVER hook two single-tuner receivers to a single DishPro Plus drop would be if someone built the "super-separator" I discussed in the other thread; it would have a DishPro Plus input and two LEGACY outputs, with circuitry more like a switch than a splitter or the official E* separator. (I doubt E* would ever build it, but I suspect MicroRoyal might make it someday if there's enough demand.) Such a device would accept legacy switch commands from each receiver and translate them to DiSEqC commands for the DPP44. (Indeed, it would have to emulate legacy switches, much like the DPP44 does with legacy receivers.) But that device wouldn't be limited to 311's; it would work with almost every single-tuner unit, including the 301.
The original statement is probably correct; the 311 is virtually the same as the 301 except for the built-in smart card. The only other difference posted at
http://ekb.dbstalk.com/101 is that the 311 can receive 8PSK channels with higher compression; but since the only non-HD models that can do that are the 311, 322 and 522, that's not significant right now. Also, since they use different firmware, the 311 may not yet have the revised user interface now being downloaded to 301's.