The security of data traffic over both SSTP & SSL
Assuming that the keys used for the SSL and SSTP layers are independent (which they should be, since both layers have their own separate key setup processes), compromising the outer layer (SSTP) cannot make the inner layer (SSL) any less secure than it would be if used alone.

To see why this is, imagine that there was an attack that did allow breaking SSL + SSTP significantly more easily than plain SSL alone.  Then an attacker could break plain SSL just as easily as SSL + SSTP, simply by taking a plain SSL datastream, tunneling it over his own SSTP connection (a trivial operation) and then breaking the combination.  But this contradicts the original assumption that breaking SSL alone would be harder than breaking SSL + SSTP.

For a more detailed analysis, see Maurer and Massey, "Cascade Ciphers: The Importance of Being First", Journal of Cryptology 6(1), pp. 55–61, 1993.