Siebel Single Instance

Hi,

We currently have 2 Siebel instances deployed to support diferent Geo, 
Is it a good idea to merge all these instances into one?
What are the challenges, organization structure, field extensions, column extensions, workflows, what should be looking at?

Is there any white papers?, best practices? and what to look at first?

We do not have major gap between business processes, can we use BPM first to streamline then work merging single repository?.

Any input / pointers would be appreciated.

Thanks.