I’m a 15th Level Unchained Rogue with Greater TWF...
My attacks are—
Mainhand +20/+15/+10 to Strike with 1D6+10 Damage
Offhand +18/+13/+8 to Strike with 1D6+5 Damage
Main hand +4 Short Sword
Offhand +2 Short Sword
What spells increase the Strike chance?
Been using Heroism but looking for more then a +2.
Not looking for equipment upgrades, just wizard spells
and Cleric spells.
Attack rather than Strike is the usual term. Or 'To Hit' among certain people who have been playing D&D far too long.
Divine favor is the obvious cleric spell, or divine power if you have money to burn. Both are personal - you can cast them from scrolls or wands using UMD but can't have them cast on you by others.
Haste gives a +1 attack bonus besides the extra attack, movement etc.
There is a greater heroism spell; shorter duration but it's +4.
Most polymorph spells can give an attack bonus, and alter self, the monstrous physique line or the giant form line (or reduce person) don't interfere with your equipment. These are mostly personal spells.
True strike is one attack only, but +20 is an all but guaranteed hit.
Perhaps the  Transformation Spell? It grants a +4 Enhancement to Strength/Dexterity/Constitution and you gain full B.A.B. - so you gain a +2 Modifier from the Enhancement and your Rogue 15's B.A.B. increases from +11/+6/+1 to +15/+10/+5, for a total increase of +6 to your Attack/"to hit" chance (before whatever other modifiers you have in play). You also get additional benefits, but the drawbacks are the price and loss of Spellcasting/U.M.D. (With a Scroll priced at 1950gp this can become taxing, yet if you have a Spellcaster with the Spell it reduces to 300gp for the Potion of  Bull's Strength Material Component.) Whilst it might seem short in duration, (1 Round/C.L.), 1 Round lost to setup leaves 10 Rounds from a Scroll which is fine for many fights; if your Wizard ally is also Character Lvl./C.L.15 you will get 15 Rounds, and so on.
To add onto @avr's post, the Fey Form line of Spells are available to Wizards, which also do not interfere with your equipment but are Personal too, so require a Scroll. However, this line gives plenty of options -  Fey Form III for the Huge Tunche will grant a Str+6 Enhancement for a +3 to Attack Rolls, whilst you can get Land (60ft.) and a Fly (30ft./Average) Speeds from  Fey Form I with the Erlking to help with manoeuvrability/the Flanking +2 Bonus instead. Go  Fey Form II to get a mix with an Ankou - Str+4 for a +2 Bonus to Attack Rolls, plus a Fly Speed of 60ft.(Good) to go up and over into Flanking position, avoiding Attacks of Opportunity on the ground.