I see that there is a rod that allows a warlock to get a bonus to hit and also to the DC of their spells. I am playing a pact blade warlock and I'm wondering if there are any magic weapons that would function as a magic weapon (longsword for example) yet also provide this benefit to warlock spellcasting?
Not in the books (yet, anwyays). Go ask your DM if your character can find or make one.
Rod = club?
Otherwise, as Aaron suggests, discuss it with your DM.
What I really want is a 2 bladed longsword that would fit this bill 
Ya, based on what is in the DMG this would need to be a special weapon, probably requiring a quest or something.
One of my players asked for a similar thing. He wanted the effects of the warlock rod in a weapon. I added the rod to the treasure list in our adventure, so he now has it. I think he still want to find a way to put the rod into the core of a sword.
Go on a quest to find a legendary master swordsmith. Ask him to show you the secrets of swordsmithing. Make a new sword, using the rod as the hilt.
This could involve such things as needing a new type of metal (that can channel magic easier .. Starmetal?) and needing to forge the blade in the fires of the most remote volcano mountain. Or, it could even involve sacrificing something of value to your Warlock patron.
I see lots of ways a DM could let this happen.
Ah, but this is the Rules Questions forum. In the current rules, there is no way to accomplish this short of DM fiat.