hello
I have just brought home my new pup and my older one (13 months) keeps trying to grap her neck with his teeth !!!!

should i just let them get on with it????

Yes, that is normal, but you will have your work cut out with the two pups, the elder one will be entering the teenage stage so you need to keep an upper hand with him and make sure all his attention is not diverted onto the younger pup. Best to walk and train them separately so they still take their clue from you.