To my knowledge, you can't fully combine both, but it is possible to partially combine them depending on the circumstances. For example, if you got the 20 000$ SSHRC fellowship AND the FRQSC (which is 21 000$), FRQSC would top off the other one. At the MA level, you can accept both and the FRQSC will fund your second year since it's a 2-year scholarship. However, it wouldn't work for the CGS-D (even if the FRQSC is longer) since the total SSHRC amount is higher than the FRQSC one.