Looking to purchase a trip that includes UA and TG. I found good webfares from UA (BKK RT) and TG (BKK onwards RT). Seperately, the two fares add up to about $2K. But when I tried the UA site for the entire routing with *A checked, the fare came back as $6K! Haven't found the combined fare on any other site.

Question:
Is it acceptable to purchase and combine tickets this way? What problems might I run into?

Thanks!