You can find hundreds of good operators for such kind of tour ...
But I would suggest You to just find a spanish guy or girl who lives in Moscow or "Piter" (as russian says) and hire him/her for the time You need.
If You want to move between the 2 city by train, remember that You need your passport in order to buy a ticket; so if You want to buy it by yourself, it is a bit tricky because they do not speak english at all. This is the only hard job You will find, actually; and a guide who lives here would easily help You buying a ticket. All the rest is easy.
Of course, if You prefer to find a tour operator, just surf on google and You will certainly find good results for "tours in spanish, in Moscow and SPB".
Good luck then.
Pietro over 6 years ago