As said the dual system was to meet the ADRs which said you had to have it (safety they say ) it was supposed to stop total brake /failure if the system lost a side, the shuttle moves across supposedly stopping the total loss ( never seen one work, like doc said undo the rear cylinder and see the pedal still goes to floor)

If thats your car in your avatar then it shouldn't have the dual system unless its a later rover, they were only fitted to later clubmans in Australia

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Roundies and Clubbies(except vans) are both ugly that is why I have Midas and a van
1293 Mini Midas Bronze,1275 Clubman van

Yes it is my car (62 Morris 850) the previous owner rebuilt/reshelled a clubman into it hence the duel system honestly the only 850 pays is the boot, bonnet and the shell .

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Respect mine and I'll respect yours.

You will probably have trouble fitting them in the normal fashion, the tin tank is in a different radial position (relative to flange) compared to a brake one. And you need to put the restrictor valve in the top of it.
Apart from that yes it'd `work' but pedal will be heavier, most brake ones are 0.70" not 0.75" like clutch ones. They move less oil in the pedal stroke than a 0.75 would and this gives more hydraulic pressure.
I would get thee a 0.70" brake one, tin or plastic- don't matter.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.