First, remember that the E* CSRs generally know less about installations than the customers do.
No one on any of these boards has even hinted about a "prototype receiver" - and believe me, if it was out there, it would be known here. The next receiver coming is the 942. HD DVR with multiple tuners.
Second, if you have a Dish 500 (2 satellites - 110/119), you have a Twin LNBF, not a Dual. If you only have a Dish 300 (single bird), it's time to upgrade.
If all you want is to be able to access cable on 2 inputs, check to see if your 4900 has a coax pass-through. I know almost nothing about those old boxes, so can't help more.
If you want to be able to see 2
satellite channels at once, then you MUST have 2 satellite tuners available - in a dual-tuner box, or by using multiple boxes.
Finally, to return to what the CSR was saying. On an E* system, you will never have less than one cable per satellite from the LNBF to the switch. With E* moving to multi-satellite (up to 5) setups, if the switch were in the box, you'd have MORE cables coming into the house. Currently, it's one cable per tuner, unless you're a single-bird user, or have a dual-tuner box along with some of the new hardware.