Maeshalls acid is peroxydisulfuric acid and its formula is H2S2O8. 

Oxidation number of peroxymonosulfuric acid (H2SO5) is  -

2 + x -10, where x is the oxidation number of sulfur

So, x = +8

However, this cannot be true as the oxidation number of S cannot exceed +6 as S has 6 electrons in its outermost shell.

This odd oxidation number of sulfur is due to the peroxy linkages involving S in H2SO5, 

So, oxidation number of S in H2SO5 is 2(+1) + x + 3(-2) + 2(-1) {for peroxy linkages} = 0

So, x = +6. So oxidation number of S in H2SO5 is +6.

Now, in Marshalls acid, which is peroxydisulfuric acid, H2S2O8, oxidation state of S is -

2(+1) + 2x + 6(-2) + 2(-1)

So, x = +6.

That is the oxidation numbers of both S in Marshalls acid is +6.