dual camera trigger
Jeffrey Warren has extensively investigated using 555 circuits to trigger cameras via CDHK. This appears to be the least expensive option, his notes are here. for completely DIY 555 circuits, see Mathew Lippincott's note here.
Cris Fastie has explored two other options providing greater degrees of control:
1) using a tiny remote control purchased on ebay. his notes and KAP forum discussions here.
2) using an AuRiCo, or AutoRigController, sold in the states by Brooks Leffler. His notes are here and here