external references

Maybe i'm missing something obvious but i'm having an issue where i create one sketch then extrude a part from that sketch.. then sketch on one of its faces to create a second part .. so far all is good.! Now if i go back to the first sketch and change the spacing of of some attachment points. on confirming these changes…

Am I supposed to build all my parts in one studio, or each part in a separate studio? It seems like it will be more difficult to model parts that fit together if I can't view the complimentary part while designing the new part. And what if I want to use the complimentary part for external reference? I looked for tutorials…