If I want to have a beer ferment in less time would doubling the amount of yeast I pitch help? Will there be any consequences of doing so? Does it matter if I make a starter for them both?
If you are already pitching the correct amount of yeast, pitching more will not make the beer ferment faster. In addition, there are negative flavor consequences to pitching too much yeast. Having proper yeast growth in the fermenter is a big factor in ester production or lack thereof. I used to pitch an entire slurry from a previous batch into the next batch. When I started using 1/3 to 1/2 of the slurry the beer flavor improved considerably. According to Dr. Clayton Cone of Lallemand, the same enzyme, Co-Enzyme A, is responsible for both cell growth and ester production and when it's doing one it won't be doing the other. Therefore, allowing for cell growth means that you will fewer esters produced and a cleaner beer. This contradicts some other theories, but it's what I've found to be true in my own brewing. You can read more about it here yeast growth and esters. So, to sum up, pitching twice as much yeast won't speed up fermentation and might have negative impact on beer flavor.
Making a starter is all about increasing your yeast cell count.
There is no reason to use more than one packet of yeast if you're making a starter. If you need more yeast, you can split the starter to get even more.
It might technically decrease the amount of time needed, but likely only by a few hours, not days, if you're normally pitching a dry packet, smack pack, or vial per 5 gallons. What you're doing by introducing a larger yeast population is ensuring that it will be dominant and healthy, very important for higher gravity beers, and probably never a bad idea.
There are several good questions already about starters:
The reason I use about a quarter packet is that yeast reproduces asexually, by fission or budding. The cells mature and divide in two. Two divide to four, then eight, sixteen, and in a day there’s billions, depending on food, trace elements like calcium, and temperature. My wort is always going like mad the next day. Using two packets over a quarter-packet saves you about four yeast reproduction cycles, or a couple hours.