Tyraniddude summed everything up quite nicely.
Tyranids never gain the bonus for having two close combat weapons, but we get to use all of our close combat weapons and our biomorphs when we attack (normally models can only use the special rules of one close combat weapon at a time) and most Tyranid models already have an Attacks value which represents our extra close combat potential.
As for Lash Whips, they only effect models who were in B2B with a Lash Whip at the start of the Fight Sub-Phase (after assault moves but before piling in).
Currently, the best way to get Rending Claws for Warriors is to buy a box of Genestealers. If you don't care about the Stealers at all, you can pilfer all 8 pairs of Rending Claws from them and you will have all you need for a full squad of Warriors. If you actually want to field the Stealers, the trick is that you can take 4 pairs of Claws for the Warriors and then use one Claw and one "mutated hand" bit for each Stealer (you will have to model them with Scything Talons, too).
That is one of the things you can keep in mind for Nids. The arms on the small/medium bugs are interchangeable with each other, and the arms for the big/huge bugs are interchangeable with each other. This can make modelling certain builds much easier. You just have to look for the boxes with the parts you want.
As a side note, the Tyranid Warrior box is actually a very old kit. I think it has been around since late 3rd edition (like 10 years old). As such, it was from the era of when boxes had nowhere near enough parts to make all the options. The more modern kits typically come with a huge number of optional parts to allow you to build any of the main options. E.g. the "new" plastic Ravener box comes with Dual ScyTals and Scytal/Rending Claws for all three models while the old metal kits had no Rending Claw option at all.
Finally, I will direct you to the Tyranid FAQ
and the main Rulebook FAQ
. These contain many updated rules and answers to common questions. Each army has their own FAQ, and you should print out a copy of at least your army's and the rulebook's FAQs to keep with you while you are gaming. Nothing worse than having a question pop up and not having the FAQ to help answer it. You can find the FAQs on Games Workshop's website by searching for "FAQ".