I think the rules of thumb that I'm starting to accumulate about Amb/Amb vs Amb/Silver is:
1) If there are 2-cost cards that you want, might as well start Amb/Amb because you can still get to 2 on an amb hand
2) If there are 5-cost cards that you really want, might as well start Amb/Silver and have reasonable odds of chance of picking up the 5-cost card and the second amb after the first shuffle
3) I think if you're P2, that's a slight point in favor of Amb/Amb because you'll get junk in your deck a turn earlier, and need to keep up with the Ambing so it doesn't spiral out of control?
4) If your opponent does not buy any ambassadors at all, and you think that the board is still Amb-friendly, then a single ambassador is probably enough, so amb/silver.
5) Conversely, if you intend to win the Amb war, and your opponent gets an Amb or two, you have to get two Ambs of your own (or an engine to play the single Amb more often), so Amb/Silver should be used to pick up a second Amb on T3 or T4, if you didn't go Amb/Amb?