You can put multiple spells on the same scroll
Magic Item Creation on Creating Scrolls on Time Required says
Scribing a scroll requires 1 day per 1,000 gp of the base price. Although an individual scroll might contain more than one spell, each spell must be scribed as a separate effort, meaning that no more than 1 spell can be scribed in a day.
So, for example, a wizard scribes one scroll of 3 spells in the same amount of time as he makes three scrolls of 1 spell. Further, because each spell's scribed individually, the cost of one 3-spell scroll and three 1-spell scrolls remains identical.
This means that there are equal mechanic reasons not to make scrolls of multiple spells (e.g. an opponent may sunder a valuable scroll) as there are to make scrolls of multiple spells (e.g. a creature can cast spells freely from a valuable scroll after taking but a single move action to retrieve the scroll).
One may struggle to convince the GM that, as other answers mention, a caster can manipulate in the heat of combat, for example, a 20-ft.-long scroll of 20 spells, but this difficulty isn't reflected in the rules. On the other hand, perhaps it needn't be; we are talking about a magic scroll, after all.
These questions were originally asked in this answer's Comments.
When employing a long scroll is a caster still limited by the spell's casting time, usually to a lone spell per standard action?
Yes. Each spell on the scroll is still cast individually. Having the spells share the same medium is for convenience, having no other mechanical effect (much like having a staff that allows the user to employ multiple spells—the spells themselves are still used individually). Casting multiple spells in a lone action is one of the wizard's holy grails, not something accomplished simply by putting the spells on a scroll (or staff).
From a scroll a caster can cast—both in the same turn—a spell modified by the feat Quickened Spell with a casting time of 1 swift action and a spell with a casting time of 1 standard action, right?
Yes. The rules permit scribing scrolls of spells modified by metamagic feats ("[Y]ou can store a metamagic version of a spell in a scroll, potion, or wand."), but the reduced casting time of the modified spell isn't a property of scrolls but of the metamagic feat.
Would a house rule permitting casters to use long scrolls to cast simultaneously multiple spells be unbalanced?
Yes. Casting spells is already the most efficient use of actions in the game. A GM who allows casters to employ scrolls to generate multiple magical effects simultaneously will find that every character who can takes the feat Scribe Scroll, that casters even more rapidly overshadow other PCs, that standard casting is obsoleted by his house rule, and that even the non-casters will invest heavily in the skill Use Magic Device so they, too, can share in the weird bounty. Even in a one-on-one campaign, the lone PC is doomed when an opponent whips out a scroll. (However, in a one-on-one campaign, I can imagine this as a campaign-specific secret technique that the lone PC is allowed to use but others creatures aren't (except, perhaps inevitably, the big bad), but the GM should expect such a character to spend all of his downtime scribing or acquiring scrolls.)