Order a maximum of 1particular product with multiple quantity at a time

[dursunj]

Hi,

 

My client wants to limit its user so that they are able to purchase 1 particular product at a time , the latter can increase its quantity and should not be able to add another different product.

 

For example, let say i have 3 products.

 

Product A

Product B

Product C

 

At any given type a user can i any product in his cart.(for example Product A), now if i wish to add Product B, he get an error, but if he want to increase the quantity for Product A, he can do that.

 

Please let me know how can that be possible...