A purely hypothetical question from an American master's student who has been in the UK since undergrad on two consecutive student visas:
Is it feasible to do two graduate courses (in my case, a research doctorate alongside a separate, second vocational MA in a neighboring city) alongside one another, in a discreet manner?
Would a student pursuing both courses be able to enter the UK on a 3-year Tier 4 student visa registered with the doctorate-granting institution, having neglected to register the CAS number offered by the MA-granting institution? Would it simply be a matter of "I'm here legally; I can pursue supplementary study for fun!" or would Hypothetical Student risk deportation and/or getting in serious trouble? The UKBA site is ambiguous - it seems to simultaneously forbid studying at multiple institutions while leaving an enormous grey area in the field of acceptable "supplementary study."
(for the purposes of this argument, let's assume that the academic-feasibility and financial-feasibility side of things have already been discussed and tacitly approved by some members of the doctorate-granting institution. Quite a few home students of my acquaintance have been able to do the "double degree" thing (usually a law degree alongside a doctorate, although in one case it was a second master's degree) without being found out; however, they didn't have visa issues involved.).
Both institutions are regarded as Highly Trusted Sponsors and are in no danger of having said privileges revoked anytime soon, if that makes a difference.