Help to buy and Lifetime ISA's

hi all,

first post here so bear with me, ive been doing some research on moving money from my Help to buy isa to lifetime isa. on the website martin has come up with a plan of.....

Rule no. 4 – Unsure when you’ll buy? Max out a Help to Buy ISA and put £1 in a LISA. Again, there’s a schedule to follow to max this out…
– Open a top Help to Buy ISA now and save to the max in it.
– On 6 April 2017 open a LISA with only £1. That way the clock has started on the ‘you must have held it a year’ rule.
– If you’ve more savings to put away, use normal top savings accounts.
– In March 2018 (before the tax year ends) if you’re not close to buying, transfer the Help to Buy ISA into the LISA. And also take any money out of normal savings to put in the LISA up to £1,600 (the LISA limit minus £2,400 – a year’s maximum Help to Buy contributions). This way you’ll have maxed out your contributions for when you buy (if you aren’t convinced you’ll definitely buy at some point, stick with a Help to Buy ISA, it’s safer).

My question is i thought you wernt allowed to pay into more than one ISA per tax year? so how can i open a LISA with £1 and continue to pay into my HTB ISA in the same tax year?

Thanks in advance for the help.