I recently lost my digital tacho so i phoned the dvla and ordered a new one.
My question is
will the card i lost be deactivated or can i still use it but keep the new one as back up?
thanks for all the replys
no it wont be deactivated there is no way of doing that,
but bear in mind if you find youre old one 
that you lost it is illegal to possess two of them as far as i am aware. it would be wise to bin the old one, you can still drive without one by doing the printout thing (one at the start and one at the end of your shift)
Let’s get some things really clear;
You have reported a card lost and obtained a replacement card for it. When you obtained the replacement card you signed a form, hope you read it, ‘cos it says that the replacement card is the one and only card in your possession. The ‘original’ card will now have a ‘marker’ put against it that says it is ‘lost.’ Should the Police / VOSA check your card against a pan-European system called Tachonet, the ‘original’ card will have a ‘lost’ marker displayed against its’ number. Now, try explaining why you are using a card which has been ‘replaced’ and you have signed a document saying that you don’t know where it is. You may not have noticed the bit in the small print telling you that ‘false records’ are an indictable offence, in other words you may by-pass Magistrates and get sentenced in Crown Court, that means a small room with a bad view out of the window 'cos the bars get in the way.
All I can advise is that you get the ‘previous’ card downloaded by your employer, if they haven’t already, cut it in half and send it back to the DVLA by recorded delivery, pronto, and say that you have just ‘found it’ 
Ignore the bit about printouts, that is only for the period between you loosing the ‘original’ card and getting the ‘replacement.’ Even then it can only be done for a maximum of 15 days and you must report the loss within 7 days.