The following VBA example creates three ellipses and two blends between them. One ellipse, referenced by the s2 variable, is part of both blends. The Effects collection is then applied to that ellipse, to access both blends and adjust their spacing-acceleration properties.
Sub Test()
Dim doc As Document
Dim s1 As Shape, s2 As Shape, s3 As Shape
Dim e As Effect
Set doc = CreateDocument
Set s1 = doc.ActiveLayer.CreateEllipse(0, 0, 2, 2)
s1.Fill.UniformColor.RGBAssign 255, 0, 0
Set s2 = doc.ActiveLayer.CreateEllipse(4, 4, 5, 5)
s2.Fill.UniformColor.RGBAssign 0, 0, 255
Set s3 = doc.ActiveLayer.CreateEllipse(0, 7, 1, 8)
s3.Fill.UniformColor.RGBAssign 0, 255, 0
s2.CreateBlend s1
s2.CreateBlend s3
For Each e In s2.Effects
e.Blend.SpacingAcceleration = -100
Next e
End Sub