Doable , certainly, possible with an existing XP CD .... maybe.
If the XP is a OEM copy which you have activated before, a problem is likely when you try and activate. If it's a retail version no problem.
I always do the install to the HD 1st. After building I'll disconnect the data cable to the SSD and install XP to the HD. Once complete, drivers installed and WU done, I'll then yank the data cable on th HD, connect the SSD and do that install.
Once done, connect the other data cable and use the BIOS boot order selector to decide which one to boot from. You can do this with 1 copy of Win 7 and , yes, when so installled on both the HD and SSD there will be no activation issues.