if the values are marked as multivalue fields, you can pass them through the transaction command using the mvlist flag. After they are both listed on the transaction, you can then use streamstats to add them even-though they are the same name.
index=iis sourcetype=iis-w3c (cs_method=POST cs_uri_stem=/page1) OR (cs_uri_stem=/page2)
| transaction c_ip maxspan=12s startswith="cs_method=POST cs_uri_stem=/pagea" endswith="cs_method=GET cs_uri_stem=/pageb" mvlist=time_taken
| streamstats sum(time_taken) as Total_Time_Taken window=1
this would mean if page1 has time_taken=1 and page2 has time_taken=2, the resulting transaction will have 2 time_taken fields, one marked 1 and one marked 2. Streamstats will then create a new field "Total_Time_Taken" and add all occurrences of "time_taken", resulting in a value of 3.
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