you should read the manuals for your ftp program to check if there is a way to preserve times.
also, the "-ot" option, well, in the manual page of the box im stading now, it dosnt specify if its creation time, or modification time.
if you have gnu date, it has a switch to return the modification time in seconds since epoch.
if you dont have it, i found a 4 lines c app that does exactly that. (it saved my day, when i needed to check time since last modification of a log file in an ancient sunOS)
general pastebin - modtime - post number 1186288
in my opinion, you should switch to scp (which can preserve time stamps)
maybe, copying the 2 files to only use one is not the best.
you could also use a HERE document with ssh to get the modification time, and only scp the one you need.
on this subject you could also use "expect"
which is a scripting language made specificly to manage interactive programs within a script.
Expect - Expect - Home Page