RE: If what you REALLY want is an E3 or Valco...
Rachel:
I think you should wait to get the E3 and in the meantime, use the P3 with or without a sling. Your kiddos will be almost 3 1/2 years apart and mine are a little over 2 1/2 years apart. We have used a double maybe 5 times for walks and never for routine errands. My older DD has learned to ride on the footboard on the P3 (which is why I got it), and she can even do it when the seat is in second to full recline position. Maybe she can even do it in full recline position. It's not easy, but she has learned.
I was using public transportation the first few weeks my new baby was here, and I put her in the Bjorn and pushed my older daughter. It was tiring, but doable. But pushing a double can be tiring too (at least the Graco tandem I have and the Maclaren side-by-side).
Another option is see if you can borrow a double from someone and see how it works for you. The common Graco tandems will usually take an infant car seat (I think). I borrowed a Graco tandem for the early infant phase and then bought a Mac side by side, but it turns out I am not really using either.
Others on these boards have liked the Aria Twin (which most people like better than the single Aria) and the Combi Twin Savvy, but the COmbi has pretty narrow seats. Only a few people have bought the Zooper Tango, to my knowledge, maybe because it's heavy and unavailable in most brick and mortar stores, so I would research that one well before plunging.
Advice and commentary on living overseas
DD1 15, DD2 12, and DS 9