I need 1300mA at 3.5V, but the highest power AC-DC wall adaptor I have is 1A 12V. Is it possible to use two of these to get ~2A? I'm thinking that diodes would be needed from the positive pins of the supplies?
1A at 12V is 12W (
P = IV). You can step the voltage down to convert this power into more amperage. Specifically, at 3.5V you could get up to 3.4A. Bear in mind that conversions will introduce power losses.
What is the load? You may not even have to limit voltage -- only current.
To answer your question directly, it depends on the type of wall wart. There is a danger when paralleling sources that one of them takes more of the load than the other. This is especially true with regulated or switching supplies, which have feedback systems to regulate their output voltage. It should not be much of an issue when using identical supplies that are a simple transformer, full-bridge and low-pass filter. They won't be perfectly matched, so don't use them to a full 200% of either's power limits. You could put current-sense resistors inline with both to measure their contribution to various loads.
You could try to parallel two wall warts with diodes, but I don't recommend it, because it requires two closely matched output voltages. And the resulting output voltage is still 11V (12V minus the diode drop). Going from 11V to 3.5V at 1300mA by means of a linear regulator will result in a 10W dissipation in the regulator. That's more than twice the power you actually need, and almost as much as one wall wart can deliver.
One wall wart can deliver 12W and you need 3.5V \$\times\$ 1.3A = 4.55W. That's only 40% of the available power, so we only need a 40% efficient conversion. Piece of cake for an SMPS (Switch-Mode Power Supply), aka switcher. This one from Linear Technology will give you even 85% efficiency, so that you'll only draw 0.45A from the 12V wall wart.
This is a typical application for 3.3V out, for 3.5V replace the 316k resistor with a 343k type.
Another well-known switcher source is National Semiconductor. Their Webench applet designs your switcher to your specifications and gives you a schematic including BOM.
You could 'solve' the need for parallel supplies by using a switch mode supply after your adapter to get the voltage down and have a respectable amount of current going out. Basically a DC-DC converter will do that, but I guess the cost of making/buying such device is more than another adapter.
You could maybe add 2 diodes in series of the output of an adapter and connect them together after that, so they won't be able to feed each other. If you're using a linear regulator after that to get down to 3.5V, that won't be much of a problem.