This is going to be difficult. I assume you have a tadpole or delta and you're trying to drive the dynamos off the paired wheels - and not the single (drive) wheel.
Problem 1: Single-Sided Hubs
The problem is that the paired wheels often use single-sided (aka wheelchair) hubs and most dynamos are made with standard hubs that are designed to be supported on both sides. Schmidt SON are (apparently) the rare counterexample that have single-sided custom versions.
Problem 2: Dynamos have a "direction"
Furthermore, dynamos are often built to rotate in one direction. Rotating them the other direction will lead to fiddly bits falling out of them. That's a problem because almost all dynos have the wires coming out one of the sides (usually the drive side) - and that'll mean that for the right side wheel, the wires coming out the right side will be unsupported. So in practicality, having a right-side dyno might be very hard.
Problem 3: Double the Drag
Dynamos have drag even when non-powered. It varies from about 3-5 watts which is negligible especially on a commuter bike, but when you have two of them, you're doubling to 6-10 watts of constant drag, which is felt more on a trike.
Problem 4: Double the Electrical Inefficiency
While you'll have twice as much current, you'll also at least double the amount of power you're losing to inefficiencies. You can rectify both using bridge rectifiers to get DC from AC but you'll lose some voltage (and thus power) going through the bridges. The problem is going around corners or in a circle, one hub will be putting out a higher voltage than the other and the lower voltage one may not be operating at max efficiency depending on your circuitry. The bridge rectifiers will prevent backfeeding but you're increasing the inefficiency of the system.
Solution 1: Use buck-boost circuits to charge Lipos
On my commuter (bicycle), a front dynohub is constantly charging a LiPo whether the lights are on or off. This is because my cheap Sanyo hub has drag regardless of whether it's under load, so I might as well as use the drag. I rectify and then pass it through a buck-boost to charge up a LiPo which then I boost to charge my cellphone when connected. You'll want parallel buck-boost circuits to avoid the problem of unequal power output listed in Problem #4 above.
Solution 2: You're on a trike, forget dynos
You're on a trike and so weight is obviously not important to you (sorry, couldn't resist the dig). So just get a small 10,000 mAh battery pack and/or solar charger and use that. It's better than the constant drag you're imposing.
Solution 3: Get one really high quality dyno
Get just one really high quality Schmidt SON dynamo and put it on the left side paired wheel if possible. Why the left? If you’re in a country where you drive on the left (most the world except UK and Japan) then the road slightly slopes to the right for drainage. You're already needing to constantly turn a bit left to maintain a straight line and the unequal drag on the left will act as a slight trim and be felt less.
The Schmidt SON dynos have been measured to have the lowest amount of drag when unloaded. Peter White says they have a tadpole compatible Schmidt SON for special order. I have no affiliation with PW except for a single order I placed a decade ago.
