Important: This is officially not supported, because you can create multiple functions with a single bot. There are not much reasons why you need a second bot on the same TS3 server.
Open the file include/botcreate.php
Add after the line 655 and 858:
$isBotMissing[0] = 0;
Line numbers was taken from Web Interface build 3033. Just search for the string $isBotMissing[0] in this file, if the line numbers don't match anymore.
Remember: You have to do this again, after you have replaced this file by an update of the Web Interface.