In the US, PT programs result in either a masters or doctorate level degree...taking 6-7 years to complete. The DC of chiropratic usually is an 6-8 semester program or 10 trimesters, but the prerequisite level of courses may or may not require completion of the BS prior to admission (I would believe most require it at this stage).
Yet, one thing to consider is that the prerequiste course work may vary for both...therefore, you might not be able to complete your PT degree and then go right to school to complete the DC...you may have to take some courses that were different between PT and DC...something you may want to consider.
What is it the draws you to the DC? Is it just the hands on approach, or do you also enjoy the aspects of nutrition, etc? If you are more interested in the manual process, why not complete your PT degree, then enter a residency program for manual therapy...Grimsby Institute, Kaiser, Paris, etc. This will give you the basis for PT as well as the manual therapy degree with a more streamlined approach.
If you really enjoy seeing the patient for primary care approach and the emphasis on nutrition, then I would just go right for the DC...especially because most DCs and PTs tend to but heads anyway.
ADDENDUM: I should edit my answer...the DC program is typically 8-10 semester/trimeters at the GRADUATE level (such as national college of chiropractice)...I didn't mean to imply that DCs receive less training than PTs...so yes, it does wind up being a 7-8 year process.