You are overwriting 'szQuery' again and again. The problem is in the amxx code, not in the query.
Try something like this:
PHP Code:
formatex( szQuery, 999, "UPDATE `users` SET `MLP` = 'MLP + 1' WHERE steam_id = '%s';", MLPsteamid);
SQL_ThreadQuery( g_hTuple, "QuerySetData", szQuery);
formatex( szQuery, 999, "UPDATE `matches` SET `MVP_steamid` = '%s', `MLP_steamid` = '%s' WHERE match_id = '%s';", MVPsteamid, MLPsteamid, szLine);
SQL_ThreadQuery( g_hTuple, "QuerySetData", szQuery);
Also I recommend you to merge the 1st and the 2nd query to a single query.
You can also try like this:
PHP Code:
formatex( szQuery, 999, "UPDATE `users` SET `MLP` = 'MLP + 1' WHERE steam_id = '%s';UPDATE `matches` SET `MVP_steamid` = '%s', `MLP_steamid` = '%s' WHERE match_id = '%s';", MLPsteamid, MVPsteamid, MLPsteamid, szLine);
SQL_ThreadQuery( g_hTuple, "QuerySetData", szQuery);
But i'm not sure if it's works in here. As far as I know, you have to pre-configure that you want to allow multiple queries and I don't know if it's possible in AMXX, but worth a try.