You are correct, you cannot pay into 2 different cash ISAs in a single tax year. However there is a way round this for you...
Pay the remaining £2400 into the HSBC ISA. Open a new cash ISA with a provider that accepts transfers in (eg. A&L Direct ISA 4). Make sure you tell them it is just for a transfer and get them to do it for you.
If you do it this way, you will get your cueent £3600 and the previous balance over to an account paying 6.25%.
This account does include a 1% bonus until 31st May 2009, so you may need to move it again then, but in the mean time, you'll get much better returns.
Hope this helps...