JeySee, what comparison between the two versions of the items do you want?
For instance, if you want to compare the item difficulties to discover which pairs have significantly different difficulties, then
1. in the item labels, put the item-context code "a1", "a2", ... "b1", "b2", ....
2. model the two versions of each item to share the same partial-credit structure
ISGROUPS = 12......12.......
3. The analysis will produce a measure, S.E., and count of observations for each item.
So, for each pair of items, we can use Excel to compute Welch's variant of Student's t-statistic: http://www.winsteps.com/winman/index.htm?t-statistics.htm