Hi
SAP BO & BW are both Independent systems, So upgrade to the systems are not interdependent.
We can go ahead upgrading the systems parallely with out any issues.But we might face few issues related to the patch levels. For which there are various solutions offered by our Gurus.
Please also refer the below discussion related to upgrade :
BO (webi) impact with BW 7.4 upgrade | SCN
Thanks
Yogi