Wire two 14-50 outlets to 50 amp breaker
On 50A circuits, the rule is one breaker - one homerun - one outlet (NEC 210.23) The only exception is circuits which supply only cooking appliances (210.23C).  If you want to use two devices on one circuit, unplug the one you're not using and plug in the one you are.

It may be legal to feed two outlets with 2 breakers in a sub-panel.  However I would think an inspector would look askew at a 50A breaker in the main panel serving two 50A breakers in a sub-panel.  

It would probably be allowed if you had an interlock which allows only one breaker to be on at a time.  Those are readily available, intended for generator interlocks.  Just backfeed them - this is allowed unless the breakers are GFCI or AFCI.  (and even if they are, move them around so they forward-feed.) 



For wirenutting stuff that big, I use screw-down terminal blocks, they are more reliable.



I wouldn't worry about liquid-tight unless you actually expect water ingress.  

I use metal conduit if the area is dry and unlikely to rust.  It qualifies as a ground path, so pulling ground wires is unnecessary.  As you may need to pull another set of 6 AWG... For 6 AWG wire, the hard max in 1" conduit is 6 wires.  Sure, that's pullable - if you have an electrician's truck full of special pulling tools.  But I don't, and I don't want to call that guy, so I wildly overbuild conduit. I use oversize conduit and lots of access points (junction boxes, conduit bodies) with no more than one 90 between them.  At that point I can usually just push the wire through, without fishing.  

If you're running conduit, don't use 6/3.  That is sheathed multi-wire cable, informally called Romex, and it's stiff and miserable to pull through conduit.  You are not allowed to remove the conductors from the sheath unless they have appropriate markings of their own, which they won't.  The right stuff to use in conduit is stranded single-wire THHN (in dry areas) or THWN (in wet areas) - at 6 AWG size, most wire will be combo THHN/THWN.

See what your equipment requires.  If you're in conduit, and it doesn't need neutral, don't put it in - you can always add it later. You'd use NEMA 6 instead of NEMA 14.  Never use NEMA 10, it is obsolete. 



Edit: I need more space to answer your comment.  

From what they said on the website, Juicebox only needs 2 wires (plus ground). It is a modern style "switching" power supply which chops any input 110-250V into what it wants. What they call "red neutral" is only neutral in a 120V hookup, in 240V it is the other hot.  So, no neutral. Saved you a wire.  It is legal for Juicebox to wire their NEMA 14 plug with no neutral, I honestly have no idea if it's legal to not connect neutral on a NEMA 14 receptacle. 

The website says two Juiceboxes have the intelligence to share a connection of limited current capacity.  That is not unprecedented.  Some washer-dryer combos are designed so the washer daisy-chains off the dryer, making it unnecessary to run a separate 120 service. The washer and dryer coordinate to not overload the 240V 30A circuit.  They are UL listed to do that if connected according to the manufacturer instructions.  Ditto Juicebox. 

Over 50 amps, plugs and sockets are not allowed, you must hardwire the electrical connections.  Check a voltage drop calculator on the web for the wire size.  The site I used called out #3 Copper or #2 aluminum assuming a 75' run. However, you'd want to run aluminum anyway, because with large wires, metal content is a big part of the cost.**. When splicing this "big stuff" you use connection blocks with screws that typically use hex keys.  They're about $10 each at your local electrical supply house.  Grats, you graduated out of the big-box stores.  Never look back.  

Breakers protect wires, so the breaker must match "the thinnest wire", i.e. The wire in the circuit with the lowest ampacity.  If that puts you between breaker sizes, you can go up to the next offered size, e.g. 80A. If you really want to run 100A wire, you can do that. 

You have high aspirations and I would run conduit to match - no smaller than 1-1/2". Use larger boxes for working room... And think really hard about where your bends and/or conduit bodies are going to be. That big wire will be stiff.

**The infamous problems with aluminum wiring pertained to an alloy now outlawed, being used for small lamp and outlet wiring. That doesn't save any money, because in small wire, little of the cost is mineral value. It was done post-war because it was simply impossible for the surviving copper mines and smelters to satisfy post-war demand.