20A & 30A dual feed into subpanel
Abosolutely not. There is no way to combine two feeders in one panel.

Look, I do this myself -- providing multiple services to a vehicle. In one case I have 23 x 120V@5A heaters, and I have a 50A/240V inlet driving 14 of  them, six on a 240V/20A connection, and 3 on a 120V/20A connection. You can drive as many as you can plug in.  

But they have to be completely firewalled. No connections. 

Feeding a 240V split-phase panel, one leg with the 30A and the other leg with the 20A, is illegal and dangerous, not least, it will burn your house down. Not the tiny house, the big house.  So that's right out. 

Do it this way instead.

First, for the 20A circuit, you fit a 20A inlet on the underside of your building. Cable a single circuit from that inlet to red-colored receptacles in multiple locations around the tiny house.  This system does not connect to the other system in any way, shape or form.  Not at all.  I would even get a red magic marker and red-stripe both side of the Romex involved so you know it's a different service. The yellow-red or gray-red Romex will be distinctive.  

Second, have the normal service panel as you intend. This is powered by only the 30A/120V TT30 circuit.  The various branch circuits come off it, etc. etc.  

If the red socket situation doesn't appeal, you can have a red "second subpanel", but once again, they are effectively two services, and the hot and neutral wires cannot contact each other or interact in any way.  I'm not a fan of this one because it's just asking for confusion. 

Cool hack: Allow feeding the red circuit from the 30A

To do this, fit a common receptacle quite close to that 20A inlet.  Power this recep from a dedicated 15A or 20A breaker in the service panel.  Now, if 20A power is not available separately, simply install a 1' long extension cord from the outlet to the inlet. Voila! 

Further cool hack: Be 120/240V ready

A main "breaker" is already mandatory.  It is only there to be a disconnect, so its amperage doesn't matter.  Go at least 50A (really, go 225A so you have enough breaker spaces, but I digress).  Cable from a 50A/240V NEMA 14-50 inlet or short cable to the main breaker.   To connect to a 30A/240V supply, use an adapter.  To connect to a 30A/120V supply, use an adapter.   These are readily available in the RV community.  Any RV shop like RV World probably has them in stock. Or, you can build your own from a couple feet of proper cordage from the electrical supply house, and the appropriate plugs and sockets. 

If you don't want to do that now, then buy the correct panel, and 6/3 cable if it'll be buried, and fit the TT30 as you planned. Then you can always upgrade later for just a few bucks, since you haven't painted yourself into a corner.  

It is illegal to serve multiple circuits of the same voltage to a building.

You cannot, for instance, run two 120V lines out to a shed, of any amperage.  You are expected to bring out one circuit of that voltage and split it with a subpanel.  However, it's allowed if the second line is switched, on a solar/powerwall/generator and the other is not, or some other utilization issue like that.  

To be clear, it's not allowed to a shed, barn, pumphouse, pool, spa, workshed, office, storage unit, gazebo, tennis court or garage.  Fortunately, a tiny house on wheels is a vehicle.